"ILS" type stations comprise three sub-stations:
the "Localiser" station supplies information on the angular error relative to the runway axis, PA1 the "Glide Path" station supplies information on the angular error relative to a glide path, PA1 the "marker" stations provide information on the distance from the runway threshold. PA1 a "power" unit supplying all the power supply voltages required by the other units, PA1 a "signal generator" unit supplying low-frequency signals, known as information signals, and radio-frequency signals, known as carrier signals. These L.F. and R.F. signals have pure sinusoidal waveforms, PA1 a "distribution" unit for distributing and mixing signals from the "generator" unit to the transmit antenna(s), PA1 a "radiator" unit comprising one or more antennas. These can be fed with one or more types of signals generated by the "generator" unit, PA1 a "sensor" unit made up of antennas juxtaposed with or at a distance from the antennas. They are called couplers, nearfield sensors or farfield sensors. The sensors are associated ith a "recombine" unit, PA1 a "monitor" unit for measuring parameters radiated by the generator, distributor and radiator units, for x channels, PA1 a "decision" unit for synthesising the x channels of the "monitor" unit to decide to shut down the station or to switch to a back-up transmitter. PA1 x(t)=A*cos(2.pi.*F0*t+.PHI..sub.-- CSB0)*[1+m1 sin(2.pi.*f1*t)+m2 sin(2.pi.*f2*t+.PHI.2)+m3 sin(2.pi.*f3*t)+m4 sin(2.pi.*f4*t)]+B*cos(2.pi.*F0*t+.PHI..sub.-- SBO0)*[m1' sin(2.pi.*f1*t)+m2' sin(2.pi.*f2*t+.PHI.2)]+C*cos(2.pi.*F1*t+.PHI..sub.-- CSB1)*[1+m1" sin(2.pi.*f1*t)+m2" sin(2.pi.*f2*t+.PHI.2)+m3" sin(2.pi.*f3*t)+m4" sin(2.pi.*f4*t)]+D*cos(2.pi.*F1*t+.PHI..sub.-- SBO1)*[m1'" sin(2.pi.*f1*t)+m2'" sin(2.pi.*f2*t+.PHI.2)] PA1 A represents the amplitude of the C+SB signal at radio frequency F0 having a phase .PHI..sub.-- CSB0, PA1 B represents the amplitude of the SBO signal at radio frequency F0 having a phase .PHI..sub.-- SBO0, PA1 C represents the amplitude of the C+SB signal at radio frequency F1 having a phase .PHI..sub.-- CSB1, PA1 D represents the amplitude of the SBO signal at radio frequency F1 having a phase .PHI..sub.-- SBO1, PA1 F0 and F1 represent the frequencies of the radio frequency signals. Hereinafter, Fn designates a radio frequency signal of any frequency, PA1 f1, f2, f3, f4 represent the frequencies of the low frequency signals modulating the radio frequency signals F0, F1 and Fn. Hereinafter, fn designates a low frequency signal of any frequency, PA1 m1, m1', m1", m1'" represent the depth of modulation of the low frequency f1, generally f1=90 Hz for ILS and 30 Hz for VOR, PA1 m2, m2', m2", m2'" represent the depth of modulation of the low frequency f2, generally f2=150 Hz for ILS and 9 960 Hz for VOR, PA1 Note that in the VOR case the signal at frequency f2 is frequency modulated and therefore has a slightly different equation, PA1 m3, m3', m3", m3'" represent the depth of modulation of the low frequency f3, generally f3=1 020 Hz for ILS and for VOR, PA1 m4, m4', m4", m4'" represent the depth of modulation of a speech signal in the 300 Hz to 3 000 Hz band, PA1 .PHI.2 represents the phase error of the LF signals at frequency f1=90 Hz and f2=150 Hz. PA1 the signals C+SB and SBO are on identical radio frequencies. A and B are at the frequency F0, C and D are at the frequency F1, and so on for the other carriers. PA1 the LF signals modulating the C+SB and SBO channels can also be at identical frequencies: f1, f2, fn, the depth of modulation being different. The receiver then sees an overall depth of modulation which prevents it distinguishing the contribution of a faulty transmitter. PA1 in the case of a plurality of carriers within the same channel, the frequency difference is intentionally limited to a few kHz to profit from the capture effect at the ILS receivers. With equipment of the present generation, this capture effect prevents identification of the transmitter which is the source of a fault. PA1 by definition, it necessitates the use of the station reference signal generated within the transmitter and not transmitted. It therefore rules out the use of the method at any distance from the station, and thus in particular in onboard radio navigation receivers. PA1 it necessitates switching a phase-shifter by several steps, and this for all the channels to be monitored, which rules out continuous operation in so-called "real time" because information is lost between switching operations and between each scanning of one channel relative to the others. PA1 to obtain all of the parameters to be measured without affecting the operation of the beacons by interrupting the radio signals transmitted to the aircraft; PA1 to locate a fault or faults, a variation or variations in parameters that has or have caused or could subsequently cause degradation of the information transmitted to users, without having to execute a complex measurement protocol; PA1 to record the components of the transmitted signals in real time and to provide alarm information in the event of violation of thresholds set by the I.C.A.O. standards and by users, so as to conform not only to the standards but also to the recommendations of the standardisation organisations; PA1 to detect the presence of jamming signals, to reduce susceptibility to jamming, thereby reducing the error rate of guidance signals and advising users of the presence of such interference, whether the sources of jamming signals are independent of the beacon or dependent on the beacon and therefore of a "multi-path" nature. PA1 the sum of the carrier (C+SB) and the signal (SBO) is demodulated by multiplication with a frequency (F) and with the same frequency in phase quadrature, PA1 the DC components representative of the part (C) of the carrier (C+SB) and the low-frequency components from the beacon representative of the SB parts of (C+SB) and (SBO) are extracted from these products, PA1 the modulus and the phase of the part C of the carrier (C+SB) are calculated, and PA1 the amplitude of the side bands (SB) and the amplitude and the phase of the signal (SBO) are deduced from the low-frequency components. PA1 a) multiplier means for demodulating the sum of the carrier (C+SB) and the signal (SBO) by multiplication with a frequency (F) and with the same frequency in phase quadrature, PA1 b) filter means for extracting from these products the DC components representative of the (C) part of the carrier (C +SB) and signal SBO, and PA1 c) means for calculating the modulus and the phase of the part C of the carrier (C+SB), the amplitude of the side bands (SB) and the amplitude and the phase of the signal (SBO) from the low-frequency components of the transmissions from the beacon.
The angular error information is measured from the difference of the depth of modulation (DDM) of two frequencies, 90 Hz and 150 Hz.
Standard or Doppler type "VOR" beacons supply information on the angular error relative to a reference tied to magnetic North. The angular error information is measured from the phase difference between a 30 Hz reference signal and a variable 30 Hz signal.
ILS and VOR beacons conform to international standards and specifications published by the International Civil Aviation Organisation (ICAO), and in particular in the documents known as "appendix 10" and "appendix 8071" volumes 1 and 2.
Radio navigation beacons (or stations) comprise a number of functional units. When associated with and adjusted to each other, these units supply guidance signals to aircraft.
The components of a beacon are as follows:
The sensor, monitor and decision unit are responsible for the "monitoring" function monitoring the integrity of the signals transmitted.
The complexity of a system of this kind requires a high level of technical knowledge on the part of personnel responsible for using and maintaining it. This personnel is also required to use many measuring instruments.
Some of the parameters for diagnosing a fault or a variation in the information transmitted by the stations are inaccessible. For example: the phase and the amplitude of SBO signals, the frequency and amplitude error of the various transmitters forming part of the beacon.
Operators must intervene physically on the connections between the functional units, which necessarily interrupts the use of the beacon by aircraft, which reduces the availability of the beacon.
The signals transmitted by the beacons on one channel are made up of a plurality of sources called C+SB, meaning carrier and sidebands, and SBO, meaning sidebands only. These radio sources must be locked to a plurality of frequencies within the same channel. The beacons are typically single-frequency or dual-frequency beacons depending on the number of radio frequencies that they transmit.
The C+SB and SBO signals are generated by forming the product of the radio frequency signals with low frequency signals and a DC component, which constitutes amplitude modulation.
To give one, non-limiting example, the characteristic equation of the guidance signal x(t) radiated by an ILS type beacon can take the following form:
in which:
The difficulty of analysing the signals stems from the fact that:
For all these reasons the modes of demodulation employed are not suitable for observation of the basic components of the signals, either within the stations or outside the stations when they are radiated by the antennas.
The methods employed at present use measurement channels characterised by quadratic demodulation. The global depth of modulation measured does not allow the contribution of a particular transmitter to be identified so that a faulty transmitter can be identified, i.e. the part stemming from the signal C+SB and the part stemming from the signal SBO. Moreover, they are unable to extract any information as to the phase of SBO relative to C+SB.
Other than quadratic demodulation, another method is used in some equipment to extract the phase and the level of the SBO signals relative to the C+SB signals. These systems operate by sampling a so-called "reference" signal at one of the transmitters of the station and adding it to the signal to be analyzed after subjecting it to a plurality of known phase-shifts. This principle has at least two major drawbacks:
In the case of a plurality of radio carriers, for example in the case of dual-frequency stations, no distinction is drawn between the information specific to each carrier. The resultant signal is proportional to the relative field level of the radio carriers. This characteristic of quadratic demodulators is used and is known as the capture effect. It enables signals to be radiated that are specific to a plurality of angular sectors around the stations. Nevertheless, if a jamming signal is present on the channel selected for the beacon, the position angular signal is degraded by the presence of the jamming signal. The jamming signal can have various origins, either a transmitter independent of the beacon, for example an FM broadcasting station, or the beacon itself, this phenomenon being known as "multipath". In the multipath situation, the receiver receives a signal made up of a signal from the "direct path" between it and the beacon and signals reflected from obstacles, known as "reflected path" signals. Current methods do not enable direct signals to be distinguished from reflected signals.
All these limitations of current generation equipment restrict the effectiveness of the beacon maintenance resources. Moreover, these limitations are also found in the devices for monitoring the integrity of the radiated signals (the "monitors") and in the onboard receivers of navigation systems, and reduce their performance.
Objects of the invention
One object of the present invention is therefore to provide a method and a device for analysing radio navigation signals that do not have the limitations of the prior art methods and devices and which, in particular, enable users:
Another aim of the present invention is to provide a method of the above kind and a device of the above kind employed for radio navigation proper in aircraft rather than only for maintaining and monitoring the integrity of ground stations.
The above aims of the invention are achieved by a method of analysing transmissions from a radio navigation beacon, these transmissions comprising at least one carrier (C+SB) comprising side bands (SB) and a side band only signal (SBO), said carrier and said signal having any phase relationship, wherein:
The multifrequency phase quadrature demodulation effected in accordance with the present invention extracts from the radio navigation signals information that is enriched by a deeper knowledge of the characteristics of the basic components of the radio navigation signals, this enriched information being used for maintaining or for monitoring the integrity of the radiation from the beacon or to guide aircraft, depending on the intended application.
For the implementation of the above method, the invention provides a device comprising: